1. Field of the Invention
This invention relates to a double-walled corrugated synthetic resin pipe.
2. Description of the Prior Art
A known single-walled corrugated synthetic resin pipe creates an undesirably great resistance to a fluid flowing therethrough. In order to improve this construction, there has been developed a double-walled corrugated pipe which comprises a straight inner pipe, and a corrugated outer pipe. Both the inner and outer pipes are axially extrusion molded in straight shapes and then the outer pipe is corrugated by vacuum suction, or by continuous blow molding. This double-walled corrugated pipe has a number of disadvantages which will be hereunder set forth:
(1) As the inner and outer pipes are axially extrusion molded, the synthetic resin is stretched axially of the pipes, so that its molecules are oriented along the axis of the pipes. Accordingly, the corrugated pipe is greater in axial strength than in circumferential strength. Its circumferential strength is, however, of greater importance in order for the pipe to withstand high external pressure as, for example, when buried in the ground. It is, therefore, necessary to design the pipe with a greater wall thickness.
(2) Both the inner and outer pipes are substantially uniform in wall thickness. In fact, it is impossible to provide any cross-sectional variation in the wall thickness. This results in a large loss of material, and makes it difficult to produce a corrugated pipe which is lighter in weight and less expensive, and which is suitable for use in a wide range of applications.
(3) The manufacture of a pipe having a large diameter of, for example, 500 mm or above requires a large and complicated molding apparatus, and involves a variety of technical difficulties. The product is, therefore, very expensive.